The present invention is generally directed to screw cutting methods, and, more particularly, to a screw cutting method capable of finely finishing the screw face. Numerically controlled machine tools frequently include the capability of cutting a screw surface by simply entering certain parameters and instructing a screw cutting cycle. In known screw cutting processes, parameters such as the finished configuration including the thread height and the amount (e.g. depth) of cutting in the first pass are instructed, and the tool route is automatically determined. In the screw cutting cycle, the workpiece is formed with a screw surface assuming the instructed shape by repeating screw cutting processes in successive passes while varying the cutting quantity (depth) little by little. The cutting quantity in each screw cutting process and the tool route are automatically determined, whereby the screw is formed into the instructed shape. FIG. 1 is an explanatory diagram illustrating one technique of cutting a taper screw. The numeral 1 represents the workpiece; and 2 designates the cutting tool. The workpiece 1 is rotated at a predetermined rotational speed during the screw cutting process.
A machining instruction which will initiate the screw cutting operation is prepared as follows: EQU G76X...Z...I...K...D...F...A...*; (1)
where G76 is the (EIA standard) preparatory function command (screw cutting), X and Z are the coordinate values at a point D, I is the radius value of the screw portion, indicative of the taper, wherein straight screw cutting is set when I=0, K is the thread height (a distance along the X-axis direction is specified), D is the amount (depth) of cut on the first pass, F is the screw lead (the amount of movement of the cutting tool in the Z-axis direction per o revolution of the workpiece), and A is the angle of the cutting edge (the thread angle). In the first screw cutting pass of the screw cutting cycle, the cutting tool 2 is moved along the path S.fwdarw.S.sub.1 .fwdarw.B.sub.1 .fwdarw.D.sub.1 .fwdarw.E.fwdarw.S in FIG. 1. Between B.sub.1 and D.sub.1, the cutting tool 2 is advanced at a feed speed based on the screw lead factor F, and screw cutting is performed with the cutting depth D. Between D.sub.1 and E and between E and S, the cutting tool 2 is fast fed and then returns to the start point of screw cutting at a high velocity.
In more detail, the cutting operation commences by first moving the tool to point S.sub.1 from point S. Note that the length of the line segment S-S.sub.1 is equal to the length of the line segment B-B.sub.1, and hence the incremental positional change here (.DELTA.Z, .DELTA.X) can be expressed as (D*tanA/2, D), where D is the depth of cut (see FIG. 2). The cutting tool can be moved to the point S.sub.1 by distributing the X, Z driving pulses on the basis of the foregoing incremental quantities. After moving in the X-axis direction by a distance given by {Xs-(X-I+K)}, where (X-I+K) is the coordinate value at the point B, the point of the cutting tool 2 reaches the point B.sub.1. Thereafter, when the cutting tool is fed along rectilinear line B.sub.1 -D.sub.1 ' with the feed speed in the Z-axis direction equal to F, the screw cutting process is carried out with a cutting depth D and a lead F. Note that the inclination of the rectilinear line B.sub.1 -D.sub.1 ' is defined by I/(Zs-Z). Hence, biaxial drive-pulse distribution is effected so that the cutting tool moves in the X-axis direction by a total distance equal to the radius value I, and also makes a (Zs-Z) movement in the Z-axis direction. As a result, the cutting tool 2 is moved along the rectilinear line B.sub.1 -D.sub.1 '. When reaching the point D.sub.1 ', the cutting tool is then moved more quickly to the point D.sub.1. Whether the cutting tool has reached the point D.sub.1 ' is detected in the following manner. The numerical control unit includes a register for storing the Z-axis component (Zs-Z) between S and D.sub.1 when initiating the screw cutting process and performs addition or subtraction depending on the moving direction when the cutting edge moves in the Z-axis direction. It can therefore be considered that the cutting tool 2 reaches the point D.sub.1, when a monitored content of the register is equalized to .gamma. which is separately given as a parameter. When the content of the memory becomes zero, the cutting tool 2 reaches the point D.sub.1. Subsequently, the cutting tool is fast-fed through the point E and returns to the point S at high speed. Thereafter, the second screw cutting pass S.fwdarw.S.sub.2 .fwdarw.B.sub.2 .fwdarw.D.sub.2 .fwdarw.E.fwdarw.S and the third pass S.fwdarw.S.sub.3 .fwdarw.B.sub.3 .fwdarw.D.sub.3 .fwdarw.E.fwdarw.S ... are carried out while varying the cutting depth. Finally, after the screw cutting process is performed on condition that .alpha. (separately given as a lo parameter, see FIG. 2) is set as the final cutting depth, a screw assuming the instructed configuration is formed in the workpiece 1.
The method of calculating the n-th cutting depth Dn in the foregoing screw cutting cycle may vary. One example will hereinafter be described. Let D be the cutting quantity in the first pass. The cutting quantity for subsequent passes is then determined as follows: ##EQU1##
When the cutting quantity Dn is determined on the basis of formula (2), the workpiece cutting quantity (volume) can be kept constant from pass to pass for a straight screw. The cutting quantity can also be kept substantially constant in a tapered screw using this scheme, so that the load on the tool 2 stays substantially the same over the course of the entire cutting operation. Note that the n-th (n=1, 2, 3 ... ) cutting area (the portion between oblique lines in FIG. 2) becomes D.sup.2 tanA/2, and is kept constant.
Where cutting is performed according to the above-described method, however, there arises a problem in that, as illustrated in FIG. 3, the thread face (marked "A") opposed to the cutting direction, i.e. the moving direction of the tool, is not finely finished due to influences exerted by the deflection of the cutting tool during its movement. The reason for this is that the value of the final cutting quantity .alpha. (the finishing allowance) is not so large, and hence scars formed on the thread face A during the prior cutting passes are not completely eliminated by the finishing pass. These scars are created largely due to the deflection of the tool during the previous passes, due to the load on the tool.